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  1. #1
    PWCToday Newbie PiranahIndustries's Avatar
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    Chicago, IL
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    1986 JS550/750 Conversion Build

    After restoring a few other standups, I tried my hand at a complete restore/conversion build over the winter. The goal of my build was to have a reliable ski, light enough to haul around without help but with enough power to throttle out of falling. So far so good! I've been riding the ski for 3 weeks now and its performed well every time.
    Hull: Red 1986 JS550
    Motor: Twin carb 750 Big Pin bored 1mm over w/ ported exhaust manifold and Mariner 650 3 Piece Pipe (RCJS conversion plate )
    Electronics: 750 from an ssxi
    Impeller: 18* Stainless Skat Trak
    2" Extended Ride Plate
    IMG_5773.JPGIMG_5774.JPGIMG_5775.JPGIMG_5776.JPG
    1987 JS550/750 Conversion
    1990 550sx - stock

  2. #2
    I dream skis Bob Gruen's Avatar
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    Jun 2013
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    Atlanta, Ga
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    503

    Re: 1986 JS550/750 Conversion Build

    Nice. The stock pump will limit your performance with a stout 750 though.

  3. #3
    PWCToday.com Is My Home Away From Home BLRider's Avatar
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    Re: 1986 JS550/750 Conversion Build

    Very nice!
    Never argue with an idiot, they will drag you down to their level then beat you with experience.

  4. #4
    PWCToday.com Is My Home Away From Home theVetteman3's Avatar
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    Re: 1986 JS550/750 Conversion Build

    Quote Originally Posted by Bob Gruen View Post
    Nice. The stock pump will limit your performance with a stout 750 though.
    I found there is some overlap between the two. A 13/18 small hub swirl was just about equal in performance to the 20 pitch I had. 13/18 c-75 big hub swirl was a tad better and 10/16 hooker better still. A poor impeller selection on the 750 pump might not have as good of performance as a perfectly set up JS or SX pump.

    What really limits things is a restrictive pipe. I haven't seen this one too often, the head pipe looks kind of small. And just as a fyi, with things optimized with that engine, and aftermarket flame arrestors, waterbox that allows good flow, ect.. you can pull a 20 pitch. We always said with a 750 single carb is a 19 and 20 is a dual carb.

    Nice job man. Keep tweaking and improving things, that's what I've been doing for 6 years now to mine.
    1985 JS550/800 "Pretty Red": Newmiller 800, Rhaas 750 pump conv. [56.3mph gps]
    2009 1100 SJ:
    Kawi 1100
    2009 SJ MINT:
    KP steering adaptor, Worx 228 intake [45.6mph gps]
    1987 WJ650:
    6M6, JD Intake Grate, 17/20, cheater pipe [42.4mph gps]
    1994 XiR: 830cc Superstock, PJS side draft, 13/18 swirl [53.7mph gps]
    1998 SXI PRO:
    3DR hull mods [TBD]



  5. #5
    PWCToday Newbie PiranahIndustries's Avatar
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    Re: 1986 JS550/750 Conversion Build

    Quote Originally Posted by theVetteman3 View Post
    I found there is some overlap between the two. A 13/18 small hub swirl was just about equal in performance to the 20 pitch I had. 13/18 c-75 big hub swirl was a tad better and 10/16 hooker better still. A poor impeller selection on the 750 pump might not have as good of performance as a perfectly set up JS or SX pump.

    What really limits things is a restrictive pipe. I haven't seen this one too often, the head pipe looks kind of small. And just as a fyi, with things optimized with that engine, and aftermarket flame arrestors, waterbox that allows good flow, ect.. you can pull a 20 pitch. We always said with a 750 single carb is a 19 and 20 is a dual carb.

    Nice job man. Keep tweaking and improving things, that's what I've been doing for 6 years now to mine.
    Thanks for the tip! I remember seeing somewhere the Mariner 3 piece was close in dimension to the PJS (The cone was a tight fit, I don't think that could be any larger/longer), but the head pipe ID may be slightly smaller. I can tell the motor could pull more than the 18*, but I bought the impeller at that pitch and wanted to try it out before having it reworked. I'm using a half-gutted stock water box, but on the next tear down, I've got a coffman's box to swap in - and I'll probably switch to a 20* then, or a 750 pump swap if I have the funds are available.
    1987 JS550/750 Conversion
    1990 550sx - stock

  6. #6
    Frequent Poster alex03's Avatar
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    Mar 2010
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    wisconsin
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    Re: 1986 JS550/750 Conversion Build

    I did the same build a few years ago but i also did a 650sx pump swap into the 550 hull its a fun machine to ride.

  7. #7
    PWCToday.com Is My Home Away From Home theVetteman3's Avatar
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    Re: 1986 JS550/750 Conversion Build

    Quote Originally Posted by PiranahIndustries View Post
    Thanks for the tip! I remember seeing somewhere the Mariner 3 piece was close in dimension to the PJS (The cone was a tight fit, I don't think that could be any larger/longer), but the head pipe ID may be slightly smaller. I can tell the motor could pull more than the 18*, but I bought the impeller at that pitch and wanted to try it out before having it reworked. I'm using a half-gutted stock water box, but on the next tear down, I've got a coffman's box to swap in - and I'll probably switch to a 20* then, or a 750 pump swap if I have the funds are available.
    Sounds good man, feel free to ask any questions youve got. I didnt try a gutted waterbox but the stock was definitely too restrictive. A supertrap is a really good piece of the puzzle.

    Btw a smaller head pipe diameter shouldnt be too big of a deal provided it isnt too small. Smaller head pipe dias are actually better for low end and torque.

    Also just saw you're in Chicago, i live in west Bucktown and keep my skis in Skokie.
    Last edited by theVetteman3; 06-13-2017 at 08:19 PM.
    1985 JS550/800 "Pretty Red": Newmiller 800, Rhaas 750 pump conv. [56.3mph gps]
    2009 1100 SJ:
    Kawi 1100
    2009 SJ MINT:
    KP steering adaptor, Worx 228 intake [45.6mph gps]
    1987 WJ650:
    6M6, JD Intake Grate, 17/20, cheater pipe [42.4mph gps]
    1994 XiR: 830cc Superstock, PJS side draft, 13/18 swirl [53.7mph gps]
    1998 SXI PRO:
    3DR hull mods [TBD]



  8. #8
    PWCToday Newbie PiranahIndustries's Avatar
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    Re: 1986 JS550/750 Conversion Build

    I live further south in Mokena, but if you ride in Portage, the lake we are on is about 30 minutes south near Three Rivers. If it would work out, it would be sweet to have an expert like you to look everything over and give me your opinion. Not sure when I'll be up next though, likely toward the end of July.

    Next thing I was going to try to improve was stability at higher speeds - I know there is more top end, but haven't been able to reach it due to instability. I just switched to a 2" extended Ocean Pro ride plate which helped keep the nose down/pump loaded, but still looking something more. I've got the rear sponsons off (as I bought the hull without) - would putting them back on help?
    1987 JS550/750 Conversion
    1990 550sx - stock

  9. #9
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: 1986 JS550/750 Conversion Build

    i cant really see how the aftermarket flame arrestors really do much other than let lots of water in the engine when the ski lays over on the right side. what you can do that will be free is to remove the rubber pieces on the side of the stock box, then on the metal screen on the inside, you can remove the rubber gasket then take 2 of the screens out, reinstall the rubber gasket and just run one screen per carb. this is what the yamaha freestyle guys do and run the stock flame arrestor because it keeps the water out. have you done anything on the pump besides just the impeller? such as bored nozzle or stubby pump cone?
    91 440; PJS Viper 7000, jetsport exhaust mani, factory full pipe, coffman waterbox,solas 17*, jetco worked 440 pump, ocean pro nozzles. shredmaster ocean rideplate and grate combo, wide tray, footholds, deep strake mod, driveshaft fill,650sx quick turn

    build thread part 1: http://www.pwctoday.com/showthread.php?t=331343
    build thread part 2: http://www.pwctoday.com/showthread.php?t=364872
    build thread part 3: http://www.pwctoday.com/showthread.php?t=396154

  10. #10
    PWCToday Regular sigraham's Avatar
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    Feb 2011
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    Hawthorn, CA, 90250
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    78

    Re: 1986 JS550/750 Conversion Build

    Some simple and cheap suggestions to try for stability at speed;

    1. Check the hull end caps are flush and straight with the hull bottom (ie. one not angled down or up more than the other)

    2. Sand a rough finish to the bottom of the hull (eg. 120grit).

    3. Try a traction vent in the rideplate. Get a stock 550SX rideplate, they are longer than the earlier 550 plates and cheap and easy to find. Then cut a traction vent hole (1" semi-circle shape - curve facing aft) 1 cm aft of where the reduction nozzle attachs to the pump vein stator.

    4. Try a top loader Intake grate.

    The 550 hulls were never really originally designed for speeds over 45mph. At speeds above this the hull running board (center flat section of hull) generates too much lift raising the tunnel strakes out of the water, gradually minimising their straight-line directional effectiveness to zero. This is when and why they get 'skatey'. The traction vent helps reduce this excessive lift. Give it a try, you'll be amazed, especially their holding power in turns.

    There are other things you can do, but the above are cheap, easy and effective to try first.

    Happy brapping!
    Last edited by sigraham; 06-17-2017 at 08:56 AM.

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